An on-vehicle air conditioner for air conditioning, comprising a refrigerating circuit having a refrigerant outside heat exchanger installed so as to perform a heat exchange with outside air and usable for both cooling and heating, a coolant circuit having a coolant outside heat exchanger installed so as to perform a heat exchange with the outside air and allowing the coolant of an engine to circulate therein, and at least one opening/closing duct for inflow air regulation installed on the forward outside and/or rearward inside of the refrigerant outside heat exchanger, wherein the opening and closing of the duct can be controlled properly according to the situations, whereby cabin cooling, cabin heating, and defrosting can be performed efficiently and sufficiently even if, for example, CO2 refrigerant is used in the cars such as hybrid cars using electricity and gasoline as energy sources and idle stop-capable cars.